1. Field of the Invention
This invention generally relates to a hub transmission for a bicycle. More specifically, the present invention relates to an internally geared bicycle hub transmission.
2. Background Information
Bicycling is becoming an increasingly more popular form of recreation as well as a means of transportation. Moreover, bicycling has become a very popular competitive sport for both amateurs and professionals. Whether the bicycle is used for recreation, transportation or competition, the bicycle industry is constantly improving the various components of the bicycle. One component that has been extensively redesigned is the bicycle drive train.
Internally-mounted multi-speed hub transmissions form part of the rear wheel of a bicycle and include a hub axle that is mounted to the bicycle frame, a driver rotatably supported by the hub axle for receiving a pedalling force, and a hub shell rotatably supported by the hub axle. A power transmission mechanism is disposed between the driver and the hub shell for communicating rotational power from the driver to the hub shell through a plurality of power transmission paths. The hub transmission further includes a clutch for actuating the power transmission mechanism to select different gear ratios.
The power transmission mechanism ordinarily includes a planetary gear mechanism including several sun gears, at least one ring gear and a planet gear carrier each of which is rotatably supported by the hub axle. Several planet gears are rotatably supported by the planet gear carrier meshing with the sun gears and the ring gears. To change the power transmission paths and the corresponding gear ratios, the aforementioned various gear components are selectively non-rotatably coupled with each other by operating the clutch.
One example of an internal hub transmission is disclosed in European Patent No. 0 679 570 B2. The internal hub transmission of this publication is one which includes a planetary gear mechanism including three sun gears, three planetary gears which mesh with the sun gears and a ring gear which meshes with the planet gears. To select different power transmission paths, it is possible to selectively lock the three sun gears by pawls which are installed in the inside-circumferential parts of the respective sun gears and which are capable of engaging with the inside surface of the respective sun gear through the operation of springs. A sleeve is inserted into the outer circumference of the hub axle which can be caused to rotate thereby coupling/decoupling the respective sun gears.
The arrangement of the pawl shifting clutch structure wherein the pawls and the sleeve are disposed between the fixed hub axle and the inside of the respective sun gears leads to a comparatively large diameter of the hub shell. Moreover, the control of the pawls by the sleeve is relatively complicated.
European Patent No. 0 679 570 B2 further discloses an internal hub transmission which includes a planetary gear mechanism including a sun gear integrally formed with the hub axle, a set of planetary gears meshing with the sun gear and a ring gear which meshes with the planetary gears. The ring gear is further coupled with the driver by a clutch. To select different gear ratios, a clutch mechanism is provided which includes a clutch member installed around the outer circumference of the hub axle so that the clutch member is free to move in an axial direction as well as in a rotational direction. The clutch member meshes with the driver and is engagable with a planetary gear carrier by axially moving the clutch member by a rod connected with the clutch member.
The ring gear which is designed to allow a movement towards the drive part caused by pushing the rod via the clutch member, when the transmission path changes from a directly coupled transmission path to a speed reducing transmission path, requires a comparatively big and heavy design which increases the weight of the internal hub gear. Moreover, a relatively strong force is required to operate the push rod when the transmission path changes from the directly-coupled transmission path to the speed-reducing transmission path.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved bicycle hub transmission. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.